Vinyl acetate monomer (VAM) is commercially produced predominately from the reaction of oxidatively coupling acetic acid with ethylene to form vinyl acetate plus water plus the waste product carbon dioxide. Traces of ethyl acetate, acetaldehyde, and further acetoxylation products are also formed. This reaction is conducted in the presence of a catalyst in a reactor containing many tubes that are typically about three centimeters in internal diameter and about six meters long. The shell side temperature of the reactor may be from about 100.degree. C. to about 180.degree. C. and the reaction pressure from about 35 psig (pounds per square inch gauge) to about 130 psig. The gas hourly space velocity (standard volumes of feed gas per volume of catalyst per hour) ranges from about 500 to about 4000 hr.sup.-1.
Commercial experience reveals that the catalyst performance decreases with respect to time; that is, the catalyst undesirably ages. In particular, carbonaceous material may form on Pd--Au VAM catalysts during their use in commercial reactors. This formation is deleterious to the subsequent performance of these catalysts.
It is believed that this aging occurs by two mechanisms. The first mechanism for aging is essentially global for the entire catalyst charge in a reaction tube and is a sintering mechanism. The sintering mechanism is produced when the active catalytic component for the reaction (in this case palladium) undergoes a metallic rearrangement with respect to time such that the amount of catalyst available for reaction at the surface formed between the catalyst metal and the reactants decreases.
The second aging mechanism results from the deposition of a component that fouls the surface of the catalyst. Fouling of the catalyst surface blocks the surface to reactant access and thus decreases the reaction rate. This catalyst fouling usually begins at the reactor inlet, and with time progresses through the reactor tube until the surface is blocked to an extent where the catalyst becomes essentially useless.